The present invention pertains generally to light beam alignment systems. More particularly, the present invention pertains to systems for focusing a laser beam to a predetermined specific point in space. The present invention is particularly, but not exclusively, useful for aligning the focal point of a laser beam with the end of an optical fiber.
As is well known, an optical fiber is an elongated ultra-pure glass fiber that includes a central core and has an outer cladding which completely surrounds the core along the length of the fiber. Importantly, the central core has a higher refractive index than its outer cladding and the optical fiber is, therefore, capable of conducting modulated light signals from one end of the fiber to the other by total internal reflection.
Optical fibers can be generally categorized as either single mode or multimode fibers. More specifically, for single mode optical fibers, the diameter of the inner core is comparable with the wavelength of the light that is being propagated. Consequently, there is only one mode of light propagation through the fiber. Multimode fibers, on the other hand, have a core diameter sufficiently larger than the wavelength of light to allow propagation of light energy in a large number of different modes. Still, in order to avoid mode dispersion and the resultant distortion of signals, the core size of an optical fiber has dimensional limitations. The consequence of all this is that optical fibers, when used for communications purposes, will typically have relatively small diameters, e.g. approximately fifty microns (50 xcexcm). While such small diameters may be advantageous for many applications, small diameters also raise issues about how best to direct light from a light source into the optical fiber for subsequent transmission through the fiber.
In order to effectively use optical fibers in communications systems it is essential there be some ability to optically interconnect the optical fiber with the light source. Heretofore, this has typically been accomplished by mechanical means. Specifically, by establishing a mechanical link between the light source and the optical fiber, it has been possible to be reasonably assured that light from the light source will enter the optical fiber. The situation changes, however, when it is necessary to establish an optical link between a light source and an optical fiber across free space. Further, the difficulty in establishing such a free space link is compounded as the distance between the light source and the optical fiber is increased. Nevertheless, there are applications wherein it may be desirable to establish a laser (light) communications link across a free space distance that may be as much as five hundred meters, or more.
In light of the above, it is an object of the present invention to provide a system for aligning a laser beam with the end of an optical fiber which is capable of directing a focused laser beam onto the inner core of an optical fiber. Another object of the present invention is to provide a system for aligning a laser beam with the end of an optical fiber which will effectively establish a laser communications link across free space through a distance in excess of several hundred meters. Yet another object of the present invention is to provide a system for aligning a laser beam with the end of an optical fiber that is simple to use, relatively easy to implement, and comparatively cost effective.
A system for aligning a laser beam with the target end of an optical fiber includes optics which will focus or direct the laser beam toward the fiber""s target end. A plurality of three or more light receptors are positioned to surround the end of the optical fiber and they are, preferably, coplanar with the end of the optical fiber. Importantly, each of the light receptors is capable of generating a light signal that is indicative of the intensity of the portion of the laser beam that is incident on the particular light receptor. For the purposes of the present invention, the light receptors can be tracking optical fibers which are juxtaposed with the target optical fiber.
For the system of the present invention, a comparator is electronically connected with each of the light sensors and is used for creating an error signal. Specifically, the error signal that is generated by the comparator is proportional to a difference between selected light signals as they are generated by the light receptors. Using standard feedback control techniques, this error signal can then be used for moving the laser beam relative to the end of the optical fiber in response to the error signal. With this controlled movement, the system is able to align the laser beam with the end of the optical fiber.
By way of example, consider a system using four light receptors which are arranged around the target end of the optical fiber in diametrically opposed pairs. Within this arrangement, all four of the light receptors and the end of the optical fiber will be located in an x-y plane. Specifically, one pair of receptors will be aligned along an x-axis, while the other pair is aligned along a y-axis. The optics of the system can then be used to locate the focal point of the laser beam in this x-y plane (i.e. the laser beam is focused onto the x-y plane in three dimensional space so that z=0). With the laser beam focal point in the x-y plane, one pair of light receptors can then be used by the comparator to position the end of the optical fiber relative to the laser beam in the x direction (i.e. x=0), while the other pair of light receptors can be used to position the end of the optical fiber relative to the laser beam in the y direction (i.e. y=0). As indicated above, this is accomplished using the error signal. More specifically, when the light signals from the pair of light receptors on the x-axis are equal, the laser beam will be centered in the x direction. Likewise, when the light signals from the pair of light receptors on the y-axis are equal, the laser beam will be centered in the y direction. Stated differently, the error signal will be a null when all of the light signals from the respective light receptors are substantially equal to each other.
Movement of the target end of the optical fiber relative to the laser beam, in order to obtain a null error signal, can be accomplished in any of several ways. First, the optical fiber itself can be moved relative to the optical system focusing the laser beam. Second, the optical fiber and the focusing optics can be mounted on a base, and the base can be moved. Third, the system""s focusing optics can be mounted on the base along with the optical fiber and the optics can include a plurality of mirrors, of which one is a secondary mirror. For this configuration the secondary mirror can be moved until the error signal is null.
In a refinement for the system of the present invention, the optics used for directing the laser beam toward the end of the target optical fiber can be configured to profile the laser beam. For the purposes of the present invention, the laser beam needs to be profiled with a high-intensity region and a low-intensity region. Specifically, the high-intensity region will be centrally located in the laser beam and the low-intensity region will be peripherally located in the laser beam to surround the high-intensity region. Importantly, the low-intensity region of the profiled laser beam must have sufficient intensity to generate the light signals that are required from the light receptors for generating the error signal.